feat(python): add interface file and other small changes

pyargus/argus/__init__.pyi

@@ -0,0 +1,30 @@
+from argus._argus import *
+
+# Names in __all__ with no definition:
+#   Add
+#   Always
+#   And
+#   BoolExpr
+#   BoolSignal
+#   Cmp
+#   ConstBool
+#   ConstFloat
+#   ConstInt
+#   ConstUInt
+#   Div
+#   Eventually
+#   FloatSignal
+#   IntSignal
+#   Mul
+#   Negate
+#   Next
+#   Not
+#   NumExpr
+#   Or
+#   Signal
+#   UnsignedIntSignal
+#   Until
+#   VarBool
+#   VarFloat
+#   VarInt
+#   VarUInt